[css3-images] cross-fade()

I think the current definition of cross-fade() in
http://dev.w3.org/csswg/css3-images/#cross-fade-function is
incorrect.  It defines cross-fade() in terms of the porter-duff over
operator, which is not symmetric.  This means that cross-fade(A, B,
30%) is different from cross-fade(B, A, 70%).

I *think*, though I'm not sure, that the right way to define
cross-fade is in terms of the plus operator described in section 4.5
of the original Porter-Duff paper:
http://keithp.com/~keithp/porterduff/

-David

-- 
L. David Baron                                 http://dbaron.org/
Mozilla Corporation                       http://www.mozilla.com/